feat(drag-drop): add function to determine whether an item is allowed to be transferred

Adds the `enterPredicate` function which allows consumers to determine whether a particular item is allowed to be moved into a drop container. This is more granular than excluding entire groups of items using `connectedTo`.

Author: crisbeto

src/cdk-experimental/drag-drop/drag.spec.ts

@@ -851,6 +851,34 @@ describe('CdkDrag', () => {
         });
       }));
 
+    it('should not be able to transfer an item that does not match the `enterPredicate`',
+      fakeAsync(() => {
+        const fixture = createComponent(ConnectedDropZones);
+
+        fixture.detectChanges();
+        fixture.componentInstance.dropInstances.forEach(d => d.enterPredicate = () => false);
+        fixture.detectChanges();
+
+        const groups = fixture.componentInstance.groupedDragItems.slice();
+        const element = groups[0][1].element.nativeElement;
+        const dropInstances = fixture.componentInstance.dropInstances.toArray();
+        const targetRect = groups[1][2].element.nativeElement.getBoundingClientRect();
+
+        dragElementViaMouse(fixture, element, targetRect.left + 1, targetRect.top + 1);
+        flush();
+        fixture.detectChanges();
+
+        const event = fixture.componentInstance.droppedSpy.calls.mostRecent().args[0];
+
+        expect(event).toBeTruthy();
+        expect(event).toEqual({
+          previousIndex: 1,
+          currentIndex: 1,
+          item: groups[0][1],
+          container: dropInstances[0],
+          previousContainer: dropInstances[0]
+        });
+      }));
 
     it('should be able to start dragging after an item has been transferred', fakeAsync(() => {
       const fixture = createComponent(ConnectedDropZones);
@@ -1038,7 +1066,7 @@ export class StandaloneDraggableWithMultipleHandles {
       <div
         *ngFor="let item of items"
         cdkDrag
-        [data]="item"
+        [cdkDragData]="item"
         style="width: 100%; height: ${ITEM_HEIGHT}px; background: red;">{{item}}</div>
     </cdk-drop>
   `
@@ -1147,15 +1175,15 @@ export class DraggableInDropZoneWithCustomPlaceholder {
       [data]="todo"
       [connectedTo]="[doneZone]"
       (dropped)="droppedSpy($event)">
-      <div *ngFor="let item of todo" cdkDrag>{{item}}</div>
+      <div [cdkDragData]="item" *ngFor="let item of todo" cdkDrag>{{item}}</div>
     </cdk-drop>
 
     <cdk-drop
       #doneZone
       [data]="done"
       [connectedTo]="[todoZone]"
       (dropped)="droppedSpy($event)">
-      <div *ngFor="let item of done" cdkDrag>{{item}}</div>
+      <div [cdkDragData]="item" *ngFor="let item of done" cdkDrag>{{item}}</div>
     </cdk-drop>
   `
 })

src/cdk-experimental/drag-drop/drag.ts

@@ -125,7 +125,7 @@ export class CdkDrag<T = any> implements OnDestroy {
   @ContentChild(CdkDragPlaceholder) _placeholderTemplate: CdkDragPlaceholder;
 
   /** Arbitrary data to attach to this drag instance. */
-  @Input() data: T;
+  @Input('cdkDragData') data: T;
 
   /** Emits when the user starts dragging the item. */
   @Output('cdkDragStarted') started: EventEmitter<CdkDragStart> = new EventEmitter<CdkDragStart>();
@@ -363,7 +363,7 @@ export class CdkDrag<T = any> implements OnDestroy {
    */
   private _updateActiveDropContainer({x, y}: Point) {
     // Drop container that draggable has been moved into.
-    const newContainer = this.dropContainer._getSiblingContainerFromPosition(x, y);
+    const newContainer = this.dropContainer._getSiblingContainerFromPosition(this, x, y);
 
     if (newContainer) {
       this._ngZone.run(() => {

src/cdk-experimental/drag-drop/drop-container.ts

@@ -51,7 +51,7 @@ export interface CdkDropContainer<T = any> {
   getItemIndex(item: CdkDrag): number;
   _sortItem(item: CdkDrag, xOffset: number, yOffset: number): void;
   _draggables: QueryList<CdkDrag>;
-  _getSiblingContainerFromPosition(x: number, y: number): CdkDropContainer | null;
+  _getSiblingContainerFromPosition(item: CdkDrag, x: number, y: number): CdkDropContainer | null;
 }
 
 /**

src/cdk-experimental/drag-drop/drop.ts

@@ -69,6 +69,12 @@ export class CdkDrop<T = any> implements OnInit, OnDestroy {
    */
   @Input() id: string = `cdk-drop-${_uniqueIdCounter++}`;
 
+  /**
+   * Function that is used to determine whether an item
+   * is allowed to be moved into a drop container.
+   */
+  @Input() enterPredicate: (drag?: CdkDrag, drop?: CdkDrop) => boolean = () => true;
+
   /** Emits when the user drops an item inside the container. */
   @Output() dropped: EventEmitter<CdkDragDrop<T, any>> = new EventEmitter<CdkDragDrop<T, any>>();
 
@@ -249,16 +255,17 @@ export class CdkDrop<T = any> implements OnInit, OnDestroy {
   /**
    * Figures out whether an item should be moved into a sibling
    * drop container, based on its current position.
+   * @param item Drag item that is being moved.
    * @param x Position of the item along the X axis.
    * @param y Position of the item along the Y axis.
    */
-  _getSiblingContainerFromPosition(x: number, y: number): CdkDrop | null {
+  _getSiblingContainerFromPosition(item: CdkDrag, x: number, y: number): CdkDrop | null {
     const result = this._positionCache.siblings.find(({clientRect}) => {
       const {top, bottom, left, right} = clientRect;
       return y >= top && y <= bottom && x >= left && x <= right;
     });
 
-    return result ? result.drop : null;
+    return result && result.drop.enterPredicate(item, this) ? result.drop : null;
   }
 
   /** Refreshes the position cache of the items and sibling containers. */